Ink jet printing processes are mainly of two types: continuous stream and drop-on-demand.
In continuous stream ink jet printing, a continuous ink stream is emitted under pressure through a nozzle. The stream breaks up into droplets at a certain distance from the nozzle. If a specific location on the recording sheet has to be printed, the individual droplets are directed to the recording sheet; otherwise, they are directed to a collecting vessel. This is done, for example, by charging unnecessary droplets in accordance with digital data signals and passing them through a static electric field, which adjusts the trajectory of these droplets in order to direct them to the collecting vessel. The inverse procedure may also be used, wherein uncharged droplets are collected in the vessel.
In the non-continuous process, or the so-called “drop-on-demand” process, a droplet is generated and expelled from the nozzle in accordance with digital data signals only in the case where a specific location on the recording sheet has to be printed.
The printing speed of modern ink jet printers is ever increasing for economic reasons. Recording sheets suitable for these printers therefore need to absorb the inks very quickly. Particularly suitable for this purpose are recording sheets containing nanoporous inorganic compounds, preferably oxides such as aluminum oxides or silicium dioxide, or oxide/hydroxides such as aluminum oxide/hydroxides. Such recording sheets are known as “nanoporous” recording sheets.
Such recording sheets available today do not meet all of the required demands. In particular, in the case where dye-based inks are used for recording, the water fastness and the diffusion fastness of images printed on these recording sheets have to be improved. In the case where pigment-based inks are used for recording, the surface gloss of images printed on these recording sheets has to be improved, because the required photo quality very often is not obtained due to an insufficient compatibility between the ink-receiving layer and the ink. Particularly disturbing are gloss differences between different parts of the image. Furthermore, the manufacturing process of the known nanoporous recording sheets is not well mastered on an industrial scale.
In many cases, unprinted recording sheets show strong yellowing during storage in cardboard boxes.
Patent application DE 10,020,346 describes a recording sheet which contains silicium dioxide prepared in the gas phase with a size of the primary particles of at most 20 nm, wherein the surface of the silicium dioxide has been modified by a treatment with polyaluminum hydroxychloride.
Patent application WO 00/20,221 describes the reaction of silicium dioxide prepared in the gas phase with aluminum chlorohydrate. The modified silicium dioxide is incorporated afterwards into an ink-receiving layer of nanoporous recording sheets for ink jet printing.
Patent application WO 02/094,573 describes the use of silicium dioxide prepared in the gas phase in recording sheets for ink jet printing, wherein the surface of the silicium dioxide has been modified by a treatment with aminoorganosilanes.
Patent application WO 01/05,599 describes the use of silicium dioxide pigments in recording sheets for ink jet printing, wherein the surface of the silicium dioxide has been modified by a treatment with cationic aminoorganosiloxanes.
Patent application EP 0,983,867 describes the use of colloidal silicium dioxide in recording sheets for ink jet printing, wherein the surface of the silicium dioxide has been modified by a treatment with silanes of general formula (R1)nSi(OR2)4-n, wherein at least one of the substituents R1 contains an amino group.
Patent application EP 1,655,348 describes the use of colloidal silicium dioxide in recording sheets for ink jet printing, wherein the surface of the silicium dioxide has been modified by a treatment with the reaction products of a compound of trivalent aluminum or of tetravalent zirconium or of a mixture thereof with at least one aminoorganosilane.
Claimed aminoorganosilanes are 3-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, (3-triethoxysilylpropyl)-diethylenetriamine, 3-aminopropyltriethoxysilane, N-(2-aminoethyl)-3-amino-propyltriethoxysilane and (3-triethoxysilylpropyl)-diethylentriamine.
The recording sheets according to the invention show a considerable improvements of nearly all their properties in comparison to recording sheets representing the state of the art, wherein the surface of the silicium dioxide has been modified by a treatment with aluminum chlorohydrate or an aminoorganosilane.